The spectacular proliferation of electronic devices, particularly computers, in modern society, both in numbers and complexity, demands that such devices satisfy ever increasing standards of reliability and serviceability to avoid degeneration into chaos. In the early days of the computer industry, relatively high failure rates and corresponding "down time" when the computer system was unavailable to perform useful work were accepted as the norm. As the industry matured, computers became more reliable and users now rely on computer systems to be available when needed. This dependence has become so profound that, for many businesses, the mere unavailability of the computer system for any appreciable length of time may cause significant commercial injury.
In the early days of the computer industry, a computer component was replaced by shutting off power to the system, replacing the component, and re-powering the system. This is, of course, a logical way to fix a toaster, but the complexity of modern computers makes this undesirable. It is not possible to simply shut off power and turn it back on as one would a light bulb. A computer system's state and data must be saved when it is powered down. Its software must be re-loaded and its state restored when it is re-powered. For a large modern computer system, these operations can take a very significant amount of time, during which the system is unavailable to its customers.
Computer manufacturers are well aware of the dependence of their customers, and have accordingly devoted considerable attention to these problems. As a result, many modern computer system have some degree of fault tolerance and are capable of concurrent maintenance. Fault tolerance means simply that a single component of the computer system may fail without bringing the entire system down although in some cases performance of the system or some other characteristic may be adversely affected. Concurrent maintenance is the capability to repair or replace some component of a computer system without shutting down the entire system so that the system can continue to operate and perform useful work although possibly in a diminished capacity while the repair is being performed. Concurrent maintenance, also called hot-swap or hot-plug, is a common service goal for the replacement of parts. A system which is both fault tolerant and capable of concurrent maintenance can, in theory, be kept running twenty-four hours a day for an indefinite length of time. In fact few, if any, systems achieve this level of reliability with respect to every component which may possibly fail.
Electronic systems frequently use backplane circuit cards for distribution of power, data signals, and/or mounting of active or passive circuit elements and connectors. Such a card typically contains multiple parallel layers for embedded circuit patterns, grounds, or power distribution. Pluggable connectors couple the backplane to other modules which make up the electronic system, such as power supply modules, storage devices, or logic cards. Often, such a backplane card acts primarily as a distribution medium for power and/or data signals from one pluggable module, also called field replaceable units (FRUs), to another. Relatively few functional components are attached directly to the backplane itself.
As modern computer systems improve in sophistication and reliability, and users come to rely with greater dependence on the continuing availability of their systems, it is increasingly important to provide concurrent maintenance capability in computer systems.
The electrical problem, however, of concurrent maintenance is that arcing on the connector pins on a pluggable module will occur as the new powered-off module is installed into the powered-up system, typically by plugging the module directly into the backplane. As the connector mates there is an arc resulting from the difference in voltage between the powered-on system and replacement module, and a resulting current spike occurs as the discharged capacitance of the new module is charged up to the level provided by the backplane. This problem is usually not severe enough to require special precautions in the case of logic level connections because the voltage, current, and capacitance levels are sufficiently low and do very little, if any, damage to the connector pins. In the case of power connections along the backplane which can be on the order of fifty volts and sixty amperes, however, the voltage and/or current and the capacitive levels are relatively high and can damage the connections.
The problem of arcing is usually addressed by limiting the current in the power path during the installation procedure. One solution has been to add an active device, usually a field effect transistor (FET), in the power path to limit voltage and current during the hot-plug. During normal operation the device is operated in saturation mode to minimize the series resistance. Typically, the inclusion of an FET to reduce the current during hot-plugging requires control logic whose complexities will vary depending upon the application.
Another solution to limit the current during concurrent maintenance is to place a device such as a thermistor in the power path. The thermistor or other device is at high impedance or is "cold" when the device is first installed. Then as power runs through the device, the thermistor heats up and its impedance dramatically decreases. During normal operation the series resistance of the thermistor or other device is as low as possible. Attention must be given to the design to ensure the impedance, both when hot and cold, is proper and that the device is always cold when hot-plugged. For instance, if the unit being serviced is installed, removed, and installed again with the thermistor hot and in a low resistance state, the protection for current limiting will be effectively circumvented. Arcing will occur that may damage the connector or otherwise affect operation. This can happen because the discharge time of the input capacitance of the unit is usually, very much shorter than the cool down time of the thermistor.
Yet a third technique to limit current during concurrent maintenance is to add impedance in the power path and later short out the impedance with a method, an example of which may include inserting a relay into the power path. This technique to short the impedance requires additional control logic. If the impedance path is implemented using a long connector pin which is the pin that first makes contact between the hot connector and the cold connector, the impedance can later be shorted out with a short connector pin in the connector which makes the last connection. The problem that occurs with this method is that there must be sufficient time to charge-up the input capacitance of the replacement part to the level of its powered up circuit path before the impedance is shorted. The time constant can be managed by controlling the velocity of the insertion so that the distance between the long connector pin and the short connector pin is traveled in no less than the capacitive charge-up time required but this technique adds mechanical complexity to the implementation.
The problem is that all of the above implementations add components, cost, failure rate, and complexity and often have a detrimental affect on efficiency. Thus there exists a need for hot plugging a connector for which can be efficiently accomplished with minimal time.